EFFECT OF TRIS ON REACTIONS CATALYZED BY HOMOSERINE DEHYDROGENASE AND GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASE

Tris buffer produced an apparent inhibition of the [rabbit muscle] homoserine dehydrogenase (EC 1.1.1.3)-catalyzed reduction of aspartic .beta.-semialdehyde and an apparent inhibition of the glyceraldehyde phosphate dehydrogenase (EC 1.2.1.9)-catalyzed oxidation of glyceraldehyde 3-phosphate. In each case, the apparent inhibition was due to a lowering of the substrate concentration as a result of a reversible reaction between the free base form of Tris and the substrate, an aldehyde. The product of the reaction was tentatively identified as an imine on the basis of its spectral properties. The inhibition of these 2 enzymatic reactions by Tris was employed to investigate the kinetics of the reaction of Tris with their substrates. Assuming that these aldehydes exist entirely as the free aldehyde in aqueous solution, equilibrium constants of 369 .+-. 12 M-1 and 68 .+-. 1.5 M-1 were determined at 25.degree. C for the reaction of the free base form of Tris with glyceraldehyde 3-phosphate and aspartic .beta.-semialdehyde, respectively. Correcting for the existence of the hydrated form of glyceraldehyde 3-phosphate in aqueous solution, an equilibrium constant of 1.1 .cntdot. 104 M-1 was obtained for the reaction of this aldehyde with the free base form of Tris. Forward and reverse direction rate constants for the reaction of Tris with glyceraldehyde 3-phosphate were pH-dependent.